re:'phase and amplitude are affected' -
I guess I need to refine my question, how predictable is the phase change of a zobel ?
i.e. can you say for an xover of given order, you will get a given amount of phase change, plus so much extra from the zobel (presumably above it's knee freq)?
( there will assumedly be some divergence between theory and reality, let's just consider it theoretically first...)

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Impedance varies with frequency, use impedance plots of your drivers and make crossover calculations using the actual impedance of the driver at the crossover frequency

Basically, if the impedance flattens out, so does the impedance phase. If one looks at some of the stereophile measurements, they show how impedance effects amplifier response. This is very real, but may be masked by how the driver itself performs.

The idea of a zobel separate from the crossover is a "cookbook" technique. Easy to explain, but only a small part of the story.... From professional designers you will see few "zobels" that merely cancel inductive rise - it is only another crossover component...to be manipulated like any other.

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